1. Field of the Invention
The invention resides in the field of electrolytic devices and more particularly relates to chlor-alkali or alkali metal chloride cells containing cation selective membranes.
2. Description of the Prior Art
The electrolysis of chlorides of monovalent cations (including lithium, sodium, potassium, rubidium, cesium, thallium and tetra methyl ammonium) with cation selective membranes is well known for the production of chlorine and the hydroxides of such cations, particularly with respect to the conversion of sodium chloride to chlorine and castic. In the sodium chloride process the electrolysis cell is divided into anode and cathode compartments by a permselective cation membrane. Brine is fed to the anode compartment and water to the cathode compartment. A voltage impressed across the cell electrodes causes the migration of sodium ions through the membrane into the cathode compartment where they combine with hydroxide ions formed from the splitting of water at the cathode to form sodium hydroxide (caustic soda). Hydrogen gas is formed at the cathode and chlorine gas at the anode. The caustic, hydrogen and chlorine may subsequently be converted to other products such as sodium hypochlorite or hydrochloric acid.
The efficiency of these cells for production of caustic and chlorine depends upon how they are operated, that is, the balancing of the chemical parameters of the cell and the internal use of the products and further how the cells are constructed, i.e. what materials are used to form the components and what system flow paths are employed.
One particular concern in attaining efficiency is the control of the pH of the brine in the anode compartment. It is desirable to maintain the level as acidic as is necessary and sufficient to inhibit the formation of sodium chlorate in the brine particularly when a recirculating brine is employed. Sodium chlorate is formed when hydroxide ions migrate from the cathode compartment through the membrane into the anode compartment. Adding hydrochloric acid to the anode compartment neutralizes the hydroxide ions and inhibits chlorate build up in a recirculating system. Such a procedure has been described in U.S. Pat. Nos. 3,948,737, Cook, Jr., et al. and elsewhere.
The present invention comprises an improvement over the above discussed prior art techniques. The overall or system chlorine evolution efficiency of such techniques is at any rate essentially limited to the cation transfer efficiency of the cation selective membrane as may be shown by the following system chemical equations:
Membrane: EQU t.sub.+ Na.sup.+ (anode).fwdarw.t.sub.+Na.sup.+ (cathode) (1) EQU (1=t.sub.+)OH.sup.- (cathode).fwdarw.(1-t.sub.+)OH.sup.- (anode) (2) PA0 Anode: EQU (1=t.sub.+)HCl+(1-t.sub.+)OH (anode).fwdarw.(1-t.sub.+)H.sub.2 O+(1-t.sub.+)Cl.sup.- ( 3) EQU Cl.sup.- .fwdarw.0.5 Cl.sub.2 +(F)e.sup.- ( 4) PA0 Cathode: EQU H.sub.2 O+(F)e.sup.- .fwdarw.OH.sup.- +0.5H.sub.2 ( 5) PA0 Hydrogen-Chlorine Burner: EQU 0.5(1-t.sub.+)Cl.sub.2 +0.5(1-t.sub.+)H.sub.2 .fwdarw.(1-t.sub.+)HCl (6) EQU t.sub.+ NaCl+t.sub.+ H.sub.2 O.fwdarw.0.5t.sub.+ Cl.sub.2 +t.sub.+NaOH+ 0.5t.sub.+ H.sub.2 ( 7) PA0 Anode: EQU (1-t.sub.+)OH.sup.- (anode)+0.5(1-t.sub.+)Cl.sub.2 .fwdarw.0.5(1-t.sub.+)OCl.sup.- +0.5(1-t.sub.+)Cl.sup.- +0.5(1-t.sub.+)H.sub.2 O (8)
(Equation (7) represents the sum of the equations.) In the above equations t.sub.+ represents the fraction of the current carried by cations passing from the anode compartment to the cathode compartment, the remainder of the current, (1-t.sub.+), being carried by hydroxide ions passing from the cathode compartment through the membrane to the anode compartment. (F) represents Faraday's constant, the quantity of electricity theoretically required to produce one gram equivalent of chlorine and e.sup.- represents an electron. It will be seen from equation (7) that although the addition of acid (equation (3)) will neutralize the hydroxide ion penetrating the membrane and inhibit chlorate formation thereby, the system efficiency for chlorine evolution is not affected. This may be seen by comparing with the following equations:
The sum of equations (1), (2), (8), (4) and (5) is: EQU t.sub.+ Na.sup.+ +0.5(1+t.sub.+)Cl.sup.- + EQU 0.5(1+t.sub.+)H.sub.2 O.fwdarw. EQU t.sub.+ NaOH+0.5t.sub.+ Cl.sub.2 + EQU 0.5(1-t.sub.+)OCl.sup.- +0.5 H.sub.2 ( 9)
The hypochlorite ion (OCl.sup.-) may decompose by one of two routes: EQU 2 OCl.sup.- .fwdarw.O.sub.2 +2Cl.sup.- ; (10)
and EQU 2 OCl.sup.- .fwdarw.ClO.sub.3.sup.- +2Cl.sup.- ( 11)
Comparing equation (7) and (9) it will be seen that the system production of chlorine is the same but that the latter system produces some hypochlorite and thereby some chlorate. The former system has the disadvantage of requiring an expensive, dangerous chlorine-hydrogen burner.
In accordance with the present invention the acidity in the anode compartment is controlled, chlorate is substantially eliminated, a hydrogen-chlorine burner is eliminated and the system chlorine efficiency is maintained. This is accomplished by utilizing in the membrane cell an anode having an oxygen evolution efficiency substantially equivalent chemically to the hydroxide transfer efficiency of the membrane. Such anode may, for example, have at least one region having a higher oxygen evolution efficiency than the remaining regions.